Focusing instrument for cameras and the like



May 11 '1926.l

G. A; MITCHELL FOCUSING INSTRUMENT PGR CAMERAS VND' THB LIKE Filed April 16 1925 di ,i

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l This invention relates to devices, broadly sion tube 14 lined with black felt as at 19,' termed finders, for accurately determining to make a li httight connection with front the ield of view of a camera or the like, and plate 15 of tie camera. caso. With the tube for :facilitating accuratel focusing of the` 1n position behind lens 11, the image is 5- camera. Although the device finds its major thrown onto the ground glass and may then 00 use in connection with motion picture cambe viewed in the telescopic arrangement.

eras, where rthe image on the film or on the Tube T contains a small double convex ground glass is small, it will be readily objective lens 20 which views the' understood that the inventionisnot neces- 'tllrQWn 0n groundv glass G. *While Iam u? sarily restricted to use with motion picture glymg a general descriltlign of the device I. 05 l cameras, but may be applied wherever a W1l1' also give typical ensions and prodevice of4 this character is desirable. .POIjtlOnso one dign that has been worked Speaking generally, the invention com- -Out andisnow inuse. Itwillbe understood prises an arrangement for both vm'agnify O f 'pOur-se that these dimensions `are not l. l 15 lng the image received Qn the ground glass, lllnllailpn 0n tle DVEDD but 'are Only 7 vand for bringing that imageV apparently given for enabling those skilled in the art A closer to the eye so that it may be more acmore readily to understand the device and' curately viewed and inspected, for givingv more able to lpractice 'the invention. This. I uniform illumination over the whole image, ObJeCflV 1BRS 20 iS 0f a focal length 01E-5:0

2 and for giving, by a simple arrangement,dif 111m. and is located about 65.5 111mfrom th u ferent powers' of magnification. This stategroundsurface of glass G.. l ment of the invention is not intended to be Behllld the -Ob'BCVBlBIIS them is next# A exhaustiveeither on the invention itself or Collective .1611's 21 which S V metallgula!! in lon its action., The nature of the invention. Olm, and 0f dimensions about 32 111mby. 25. its objects, and its accomplishments, will' 39 mm- (the Same proportional shape as the' be best understood from the following de- Standard pidllie area inmotion-pictures, tailed description of apreferred and specific but `SOIDePVllaill larger than the ptur f form of dovioo that ombodes the invention area). Thiswllective lens 21iSaplan0-0n e so, andis illustrative thereof; reference for "8X lens and its frward plane Sdeislocatod this purpose being had t0 the accompanying about 99.9 mm. from Objective lens 20, (that u f drawings in which; l l y 1 's, from the rear face of lens 20). The ob- Y Figlisadiagram, in plan and horizontal letlv lens llrOWS 8 Somewhat enla l section, showing the application of mv deviceV Image Onto the plane fa 0f th 'OOHOIVQ toV a motion picture camera box: Fig. 2 is leDS- The, behind the 0011005176 16118 21 u a side elevation of the same; Fig. 3 is an there are two plano-c0nvex eyepiece lenses enlarged longitudinal 'section on line 31-3 22 and 23.V LEDS 22 iS Set 1n l mOuDD of Fig. 1 .showing the parts in position for 24 SWUDg 0D a Shaft 25 that may be movl the higher magnification: Fig. 4 is a similar by Swinging handle 26 Shown ill Fg 1, b0- vew showing the parts in position for the Swing lens 22 between the two positions lower magnification; and Fig. 5 is a detail ShOWl in Figs. 3 and 4. Eye pieoelens 23 9' section on line 5 5 of Fig.' 4; is mounted in a sleeve 27 that may be'moved I show at 10 a camera. case, :is for 9, m0. longitudinally between the two positions tion picture camera; showing the lens at-ll Shown in Figs. 3 and 4. When lens 22 and indicating .the normal position of the is in the postlon shown in Fig. 3 the space 5 film behind tho lons at F, The tubo 'r of between tho lens 21 and lens 22 is about 335.19. l my'device is mounted on the side of case 10; 'mnL while the space between the lenses 22 and means are'provided for so shifting the and 23 is about 1 mm. The focal length of parts that the ground glassG may be put the collective lens 21 is about 69 mm. and into the position that film F normally oceach of the lenses 22 and 23 is about 88 mm. copies, so that the same image is thrown on 'When thetwo lenses are in the position 1N. the ground glass that is Ithrown on the film. shox-:n in Fig. 3 the magnifying power ia In the particular arrangement shown in the about tc'n times: and the image viewed bythe drawings I accomplish this shift by shifting eye is apparently about in the plane of col- ,f the camera case 10 sideways on Ways 13. lectivelens 21. Then the lenses are in the 5 The iront end of telescope Tghas an extenposition shown in Fig. 4 with lens 22 swung no out ofthe way, lens 23 is movedl outwardly to its focusing position, vthe magnifying `power is about five times, the image showing apparently about in the' plane of collective lens 21. The size, and the rectangular shape,

-` of lens 21'are such as to make that lens take the whole of the rectangular image.A At

the lower magnifying power, using only lens be viewed. The rectangular lens 21 furof the rectangular'image, illuminating the corners of the image as well as the central the vertical or smaller dimension ofY theart. When the higher magnifying power 1 s used, 4the field is limited to afci'rcular field whose diameter is a little less than rectangular image.

In most motion picture 4cameras the image as thrown on the. film or as thrown onto aground glass is invert-ed'. My instrument arrangement erects the image as well asmagnifiesit. v

.The focal length of the combined lenses 22 and 23 is of course somewhat shorter than- 'the focal length ofeither lens alone., The

' position of the two lenses for high power vx'r.a.'gnification has'bee given; for low power magnification the lens 23 is drawn out to the vposition shown in Fig, 4; the distance from collective lens 21 to lens 23 then being about (59.9 mm. The position of lensA 23 'will of course depend somewhat upon obtaining the4 correct focus for the individual eye.

The lenses, excepting rectangular lens 2.1,v are circular. Their comparative diametess. are proportionately illustrated inthe draw o The ,function o f the ground' glass is toreceive the imagefrom the photographic lens 11--to receive thesame image that is afterwards to be received on the film. This image 4is real and therefore can be just as well seen through the instrument whether yor, not .the

'ground glass is used, but in practice the .marked the exact position ofthe eye piece that gives correct' focus, for his own eye, of an image on collective lens 21, then with the eye piece in that exact .position he knows 'that all lack of focus of a scene is due to the photographic lens not'l being properly focused in the plane of the film orof the i ground'glass Being ableto see the image at that plane, whether the ground glass is used or not, he may bring lens 11 to proper focus by simple adjustment-of that Fens. The ground glass, although useful, may therefore be dispensedfwit Likewise,the collective lens 21 is not absolutely necessary as the image formed by objective 20 may be viewed directly through the eye But the collectivelens is preferred as it helps not only in the magnication but also in the uniform illumination of the finally viewed image.` v

Having described a preferred: form of my invention, I claim:

.1. A focusing instrument, comprising a.

rectangular ground glass, an' objective lens, 1' ay rectangular collective lens onto which' the objective throws an image lof the ground glass, and an eye-piece arranged for viewing the image on the collective lens.A

f 2. A focusing instrument, comprising a i ground glass,- an objective lens, -a planoconvex collective lens onto whose plane surface the objective throws an image of the ground glass, and an eyespiece arranged for l n viewing the image on the collective lens.

3. A focusing instrument, comprising a .rectangular ground glass, an objective lens,

a rectangular collective lens onto which the objective throws an image ofthe ground glass, and an eyepiece, comprising two eye'- piece lenses adapted to be used together or one alone arranged for on the collective lens.

4. A focusing comprising l rectangular ground glass, an 'objective lens, a rectangular collective Alens onto which the objective throws an'image of vthe ground glam, and an eyepiec'e arranged for viewing the image on the collective' lens and comviewing the prising two eye-piece 'lenses adapted to bev l used together or one, alone, `a laterally shifting mounting for one eye-piece lens and a'. longitudinally shifting mounting for the,

other. Y

5. A focusing instrument, comprising: ground glass; an objective lens, a collective lens onto which the. objective throws an image of the ground glass, and an eye piece the like` comprising an objective, lens adapt ed to view an image cast by a photographic lens. a collective lens arranged behind the obviective and onto which the objective throws its image of the photographic lens image. and an eye-piece arranged for viewing.- the imagev onthe collective lens and comprising two eye-piece lenses adapted to be used togather or one alone.

7. A focusing instrument for a camera. or the like. comprising an objective lens adapted to view an image cast by a photographic lons. a collective lens arranged behind the objective and onto which the objective throws its image of the photographic lens image, and eye-piece a for viewthe image on the collective ens and com- 1,584,186 I' la pie lenses being less than their fwd ai-4 tance apart whenthe second mentioned one Vl0 l is Qn the optical axis. l In Witness that I claim the fo lng I have hereunto subscribed my names 3rd.

day of April, 1923.

-GEQRGE L l' 

